1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic or electrostatic type such as a copying machine, a laser beam printer and the like for visualizing a latent image formed on an image bearing member by adhering developing agent to the latent image.
2. Related Background Art
In image forming apparatuses, an electrostatic latent image formed on a surface of a moving image bearing member is developed by a developing means as a toner image which is in turn transferred onto a transfer material. Residual toner which has not been transferred to the transfer material and remains on the surface of the image bearing member is cleaned and removed by a cleaning device. In such a cleaning device, a cleaning blade made of elastic material such as rubber has been widely used as a cleaning means for removing the residual toner, for the reason that the cleaning device using the cleaning blade can be made simpler, compact and inexpensive. As material of the cleaning blade, polyurethane rubber has mainly been used in consideration of medical resistance, anti-wear, forming ability and mechanical strength.
In the cleaning device having the cleaning blade, there is an arrangement in which the cleaning blade is urged against the surface of the image bearing member from a counter direction. In a cleaning action of this arrangement, when the cleaning blade is urged against the surface of the image bearing member with a force (5 to 40 gf/cm) required for removing the residual toner from the surface of the image bearing member, at a contact portion between an edge portion of the cleaning blade and the image bearing member, first of all, the edge portion of the cleaning blade closely contacted with the surface of the image bearing member is deformed (deviation deformation or compression deformation) in an advancing direction of the image bearing member by a frictional force acting on the contact portion, and then, energy accumulated in the edge portion of the cleaning blade due to stress acts as a restoring force (repelling elastic force) to return the blade to its original condition (so-called stick-slip movement) as shown in FIGS. 6A and 6B.
From the aforementioned explanation, in the cleaning devices using the cleaning blade, the cleaning ability is determined by an amplitude and a frequency of a vibration movement effected by the energy accumulated in the edge portion of the cleaning blade, i.e., the stick-slip movement of the edge portion of the cleaning blade. Further, ideally, for example, in case of a cylindrical image bearing member (photosensitive drum), it is preferable that the vibration movement of the edge portion of the cleaning blade is limited to occur in a tangential plane of the cylinder.
The amplitude and frequency of the stick-slip movement are optimized by adjusting the coefficient of friction of the contact portion between the edge portion of the cleaning blade and the surface of the image bearing member, the configuration of the cleaning blade, and the properties (Young's modulus, Poisson's ratio and modulus (stress-strain curve)) of the material of the cleaning blade.
In such a cleaning device, even if the above-mentioned optimization is effected under an initial condition, for example, when the coefficient of friction of the surface of the image bearing member is increased or when the cleaning blade is permanently deformed due to hydrolysis, the state of the stick-slip movement of the edge portion of the cleaning blade is changed, thereby giving rise to various problems (refer to FIGS. 7A to 7C).
Firstly, as the coefficient of friction of the surface of the image bearing member is increased, for example, by adhering the toner to such surface, the frictional force between the edge portion of the cleaning blade and the image bearing member (contact portion) is naturally increased (i.e., the apparent abut force is increased; refer to FIG. 7C). Thus, the energy accumulated in the edge portion of the cleaning blade in the stick-slip condition is increased, so that the amplitude of the stick-slip movement becomes greater than a suitable value and the frequency of the stick-slip movement becomes smaller than a suitable value. As this phenomenon increases, the edge portion of the cleaning blade jumps up without following to the surface of the image bearing member, thereby causing toner to escape, toner adhesion to the surface of the image bearing member (toner fusion or filming), abnormal noise (vibration noise of blade), abnormal vibration (tremble), so-called blade take-off (in which the blade edge portion is reversed along the rotational direction of the image bearing member), and/or, damage of the edge portion of the cleaning blade and/or the surface of the image bearing member (tearing of the blade edge, scratching of the surface of the image bearing member).
In order to solve such problems, conventionally, the frictional force has been reduced by coating, on the contact portion between the tip end of the cleaning blade and the image bearing member, solid powder (lubricating agent) of inorganic substance such as graphite, boron nitride, molybdenum disulfate, tungsten disulfate or silicon dioxide, or, solid powder (lubricating agent) of organic substance such as fluororesin, silicone resin, polyamide (nylon resin), polyacetal, polyethylene or polyimide. However, as the apparatus is used for a long time the lubricating agent begins to disappear from the edge portion of the cleaning blade, since the frictional force is increased again, such a coating method is not a complete solution for reducing the frictional force. Further, although various apparatuses for continuously supplying the lubricating agent to the edge portion of the cleaning blade have been proposed, such cleaning apparatuses are complicated and expensive, and, thus, have not yet been put to practical use.
Further, in the past, OPC (organic photo semi-conductor) photosensitive drums having a surface layer using polycarbonate as binder resin have widely been used as the image bearing members. Among them, in some photosensitive drums, a protection layer (OCL) is formed by dispersing a suitable amount (3 to 40 wt %) of Teflon resin in the polycarbonate binder resin on the surface of the photosensitive drum in order to solve the above problem. By using the photosensitive drum having the protection layer (OCL) as an outermost layer and by adding inorganic fine particles (having a diameter of 1 .mu.m or less) of strontium titanate, cerium oxide, alumina or zirconia (surfaces of which are subjected to hydrophobic treatment) to the toner, such inorganic fine particles are accumulated on the contact portion between the edge portion of the cleaning blade and the image bearing member, so that the Teflon resin included in OCL is supplied to the contact portion when the OCL surface is polished, thereby promoting the lubricating effect. However, when a large number of images using a very small amount of toner are copied continuously, as the number of copies is increased, the amount of the inorganic fine particles for providing the polishing effect on the contact portion between the edge portion of the cleaning blade and the image bearing member is greatly reduced, thereby causing the abnormal vibration and blade take-off.
Further, as a method for reducing the frictional force between the cleaning blade and the image bearing member, there has been proposed a technique in which a cleaning blade coated by a nylon resin layer (referred to as "nylon coat blade" hereinafter) is used to be contacted with the image bearing member. When such a nylon coat blade is used, a frictional force between an edge portion of the nylon coat blade and the image bearing member can be sufficiently reduced. However, unlike polyurethane, since the nylon resin has no elastomer property, it is considered that a cleaning action (due to the stick-slip movement of the edge portion of the cleaning blade) for removing the residual toner is not effected but the residual toner is blocked to scrape the toner. Thus, the abut force of the cleaning blade against the surface of the image bearing member must be increased considerably in comparison with polyurethane (about two times in comparison with polyurethane), so that an abrasion amount of the surface of the image bearing member (caused by the cleaning blade) is increased and/or the surface of the image bearing member is damaged, thereby shortening the service life of the image bearing member.
Secondly, for example, if the cleaning blade is permanently deformed by hydrolysis, the abut force of the cleaning blade against the surface of the image bearing member is decreased, so that the frictional force (on the contact portion) between the edge portion of the cleaning blade and the surface of the image bearing member is reduced (refer to FIG. 7A). Thus, the energy accumulated in the edge portion of the cleaning blade in the stick-slip condition is reduced, so that the amplitude of the stick-slip movement becomes smaller than the suitable value and the frequency of the stick-slip movement becomes greater than the suitable value. When this phenomenon is grown, the edge portion of the cleaning blade does not move (vibrate) on the surface of the image bearing member not to remove the residual toner completely. Further, escape of toner may occur and/or the surface of the image bearing member may be damaged by the toner accumulated and solidified on the edge portion of the cleaning blade.
In order to solve the above problem, it is required that the use time period of the cleaning blade is determined from permanent deformation tests of the cleaning blades under a high temperature/humidity condition and that, when the use time period is expired, the cleaning blade is exchanged to a new one.
However, in this case, actually, even the cleaning blade still having the service life has been exchanged, thereby increasing the running cost. Further, in this case, when the cleaning blade alone is exchanged, the contact between the new cleaning blade and the still used image bearing member does not become familiar, thereby causing toner to escape, damage to the surface of the image bearing member and/or blade take-off.
As mentioned above, the stick-slip movement utilizing the frictional force (abut force of the edge portion of the cleaning blade against the surface of the image bearing member) on the contact portion between the edge portion of the cleaning blade and the surface of the image bearing member is quite unstable for endurance, which may result in damage of the cleaning blade and the image bearing member.
In FIG. 7B, regarding the cleaning vibration, the stick-slip movement of the blade edge is optimized. Further, regarding the up/down vibration, the blade edge is almost not moved up and down.
However, in FIG. 7A, regarding the cleaning vibration, since the abut force is small, the stick-slip movement of the blade edge becomes small, thereby worsening the cleaning ability. Further, regarding the up/down vibration, since the abut force is small, the blade edge is moved up and down on the surface of the image bearing member, thereby affecting a bad influence upon the cleaning ability. If the cleaning ability is worsened, there arise problems such as escape of toner, lateral stripes, scratch and toner fusion.
In FIG. 7C, regarding the cleaning vibration, since the abut force is too great, the stick-slip movement of the blade edge becomes unstable, thereby causing the abnormal vibration and abnormal noise. Further, regarding the up/down vibration, since the abut force is too great, the blade edge is jumped up to vibrate up and down on the surface of the image bearing member, thereby affecting a bad influence upon the cleaning ability. As a result, there arise problems such as abnormal vibration of the blade, blade take-off, abnormal noise of the blade, lateral stripes, scratching and toner fusion.